One class of opto-electrical devices is that using an organic material for light emission or detection. The basic structure of these devices is a light emissive organic layer, for instance a film of a poly (p-phenylenevinylene) (“PPV”) or polyfluorene, sandwiched between a cathode for injecting negative charge carriers (electrons) and an anode for injecting positive charge carriers (holes) into the organic layer. The electrons and holes combine in the organic layer generating photons. In W090/13148 the organic light- emissive material is a conjugated polymer. In U.S. Pat. No. 4,539,507 the organic light-emissive material is of the class known as small molecule materials, such as (8-hydroxyquinoline) aluminum (“Alq3”). In a practical device one of the electrodes is transparent, to allow the photons to escape the device.
A typical organic light-emissive device (“OLED”) is fabricated on a glass or plastic substrate coated with a transparent anode such as indium-tin-oxide (“ITO”). A layer of a thin film of at least one electroluminescent organic material covers the first electrode. Finally, a cathode covers the layer of electroluminescent organic material. The cathode is typically a metal or alloy and may comprise a single layer, such as aluminum, or a plurality of layers such as calcium and aluminum. In operation, holes are injected into the device through the anode and electrons are injected into the device through the cathode. The holes and electrons combine in the organic electroluminescent layer to form an exciton which then undergoes radiative decay to give light.
These devices have great potential for display and lighting applications. However, there are several significant problems. One is to make the device efficient, particularly as measured by its external power efficiency and its external quantum efficiency. Another is to reduce the voltage at which peak efficiency is obtained. Another is to stabilize the voltage characteristics of the device over time. Another is to increase the lifetime of the device.
Conjugated polymers may be formed by a metal-catalyzed polymerization reaction which operate via a “metal insertion” wherein the metal atom of a metal complex catalyst is inserted between an aryl group and a leaving group of a monomer. By this process, aromatic monomers comprising two or more reactive leaving groups can be polymerized to form chains of aromatic repeat units. Examples of such polymerization techniques are Suzuki polymerization as described in, for example, WO 00/53656 and Yamamoto polymerization as described in, for example, T. Yamamoto, “Electrically Conducting And Thermally Stable pi -Conjugated Poly(arylene)s Prepared by Organometallic Processes”, Progress in Polymer Science 1993, 17, 1153-1205. In the case of Yamamoto polymerization, a nickel complex catalyst is used; in the case of Suzuki polymerization, a palladium complex catalyst is used.
For example, in the synthesis of a linear polymer by Yamamoto polymerization, a monomer having two reactive halogen groups is used. Similarly, according to the method of Suzuki polymerization, at least one reactive group is a boron derivative group such as a boronic acid or boronic ester and the other reactive group is a halogen. Preferred halogens are chlorine, bromine and iodine, most preferably bromine.
Suzuki polymerization may be used to prepare homopolymers and regioregular, block and random copolymers (“copolymer” as used herein means a polymer comprising two or more different repeat units). In particular, homopolymers or random copolymers may be prepared when one reactive group is a halogen and the other reactive group is a boron derivative group. Alternatively, block or regioregular, in particular AB, copolymers may be prepared when both reactive groups of a first monomer are boron and both reactive groups of a second monomer are halogen.
The repeat units of the polymer may be selected to tune the charge transporting and electroluminescent properties of the polymer. One widely used class of repeat units are amines, in particular triarylamines, as disclosed in, for example, WO 99/54385. Triarylamine repeat units may be used to provide both blue emission and hole transporting functionality, however the present inventors have found that polymerization of triarylamine-containing monomers can be quite slow.
Attaching aromatic groups to an aromatic repeat unit will typically result in extending the conjugation of the unit across the polymer, which in turn will shift the color of emission of the repeat unit towards shorter wavelengths (or, in other words, towards a smaller HOMO-LUMO bandgap). For example, U.S. Pat. No. 7,348,428 discloses polymers formed by polymerizing monomers having the formula:X1-Ar1—[itriarylamine]—Ar2-X2 
wherein X1 and X2 are the same or different polymerizable groups, and wherein Ar1 and Ar2 are the same or different substituted or unsubstituted aryl or heteroaryl groups. U.S. Pat. No. 7,348,428 discloses in particular monomers wherein Ar1 and Ar2 are thiophene in order to obtain green emission, rather than blue emission, from the resulting polymer.
One object of the present invention is to provide polymers, in particular blue polymers, having a long half-life (that is, the time taken for the luminance of an emitter to halve at constant current). A further object of the invention is to provide polymers having high efficiency.